It really depends on your area. Around here, some can cost well over $1000 for the night. Ours was around $700 for unlimited time up until midnight, and we were really happy with him.
In my area, a reasonably-priced DJ is about $1000-$1500 for an evening. If you ask on your local boards, they might be able to give you an estimate for your area, or you can just shop around and see what reputable companies (i.e. not people off craigslist) seem to be charging, and work from there.
Re: DJ